CS130 : Computer Graphics

Tamar ShinarComputer Science & Engineering

UC Riverside

Raster Devices and Images

Raster Devices

Raster Display

Hearn, Baker, Carithers

Transmissive vs. Emissive Display

[H&

B, Fig. 2-16]

+_

ON ONOFF

LEDs

anode

cathodes

LCD LED

Raster Display

red, green, blue subpixels

What is an image?

Continuous image

Raster Image

A raster image is 2D array storing pixel values at each pixel

What is an image?

Raster image

= number of columns

= number of rows

Each pixel value represents the average color of the image over that pixel’s area.

What is an image?

Raster image

= number of columns

= number of rows

Each pixel value represents the average color of the image over that pixel’s area.

RGB CMYK

Color Representation

sRGB color triangle comparison of color gamuts

[wikipedia]

Color Representation

Alpha Channel

Graphics Pipeline

Modern graphics system[A

ngel and Shreiner]

17

Z-buffer Rendering

•Z-buffering is very common approach, also often accelerated with hardware

•OpenGL is based on this approach

3D Polygons Image PixelsGRAPHICS PIPELINE

Choice of primitives

• Which primitives should an API contain?

• small set - supported by hardware, or

• lots of primitives - convenient for user

Choice of primitives

• Which primitives should an API contain?

➡small set - supported by hardware

• lots of primitives - convenient for user

Choice of primitives

• Which primitives should an API contain?

➡small set - supported by hardware

• lots of primitives - convenient for user

Performance is in 10s millions polygons/secportability, hardware support key

Choice of primitives

• Which primitives should an API contain?

➡small set - supported by hardware

• lots of primitives - convenient for user

GPUs are optimized for points, lines, and triangles

Choice of primitives

Other geometric shapes will be built out of these

• Which primitives should an API contain?

➡small set - supported by hardware

• lots of primitives - convenient for user

GPUs are optimized for points, lines, and triangles

Two classes of primitives [A

ngel and Shreiner]

Geometric : points, lines, polygonsImage : arrays of pixels

ff

Point and line segment types [A

ngel and Shreiner]

Polygons• Multi-sided planar element composed of edges and

vertices.• Vertices (singular: vertex) are represented by points • Edges connect vertices as line segments

E1

E3 E2

(x1,y1)

(x2,y2)

(x3,y3)

Valid polygons

• Simple

• Convex

• Flat

Valid polygons

• Simple

• Convex

• Flat

OpenGL polygons

• Only triangles are supported (in latest versions)

GL_POINTS GL_TRIANGLES

GL_TRIANGLE_STRIP

GL_TRIANGLE_FAN

Other polygons

triangulation

Graphics Pipeline[A

ngel and Shreiner]

ff

Pipelining operations

* +ba

c

An arithmetic pipeline that computes c+(a*b)

3D graphics pipeline

Vertex processor

Clipper and primitive assembler

RasterizerFragment processor

Vertices Pixels

Geometry: primitives - made of verticesVertex processing: coordinate transformations and colorClipping and primitive assembly: output is a set of primitivesRasterization: output is a set of fragments for each primitiveFragment processing: update pixels in the frame buffer

Graphics Pipeline (slides courtesy K. Fatahalian)

Vertex processing

v0

v1

v2

v3

v4

v5

Vertices

Vertices are transformed into “screen space”

Vertex processor

Clipper and primitive assembler

RasterizerFragment processor

Vertices Pixels

Vertex processing

v0

v1

v2

v3

v4

v5

Vertices

Vertices are transformed into “screen space”

EACH VERTEX IS TRANSFORMED

INDEPENDENTLY

Vertex processor

Clipper and primitive assembler

RasterizerFragment processor

Vertices Pixels

Primitive processing

v0

v1

v2

v3

v4

v5

Vertices

v0

v1

v2

v3

v4

v5

Primitives (triangles)

Then organized into primitives that are clipped and culled…

Vertex processor

Clipper and primitive assembler

RasterizerFragment processor

Vertices Pixels

Rasterization

Primitives are rasterized into “pixel fragments”

Fragments

Vertex processor

Clipper and primitive assembler

RasterizerFragment processor

Vertices Pixels

Vertex processor

Clipper and primitive assembler

RasterizerFragment processor

Vertices Pixels

Rasterization

Primitives are rasterized into “pixel fragments”

EACH PRIMITIVE IS RASTERIZED INDEPENDENTLY

Fragment processing

Shaded fragments

Fragments are shaded to compute a color at each pixel

Vertex processor

Clipper and primitive assembler

RasterizerFragment processor

Vertices Pixels

Vertex processor

Clipper and primitive assembler

RasterizerFragment processor

Vertices Pixels

Fragment processing

EACH FRAGMENT IS PROCESSED INDEPENDENTLY

Fragments are shaded to compute a color at each pixel

Pixel operations

Pixels

Fragments are blended into the frame buffer at their pixel locations (z-buffer determines visibility)

Pipeline entities

v0

v1

v2

v3

v4

v5 v0

v1

v2

v3

v4

v5

Vertices Primitives Fragments

Pixels Fragments (shaded)

Graphics pipeline

Primitive Generation

Vertex Generation

Vertex Processing

Fragment Generation

Fragment Processing

Pixel Operations

Fixed-function

Programmable

Memory Buffers Vertex Data Buffers

Textures

Output image (pixels)

Textures

Textures

Primitive Processing

Vertex stream

Vertex stream

Primitive stream

Primitive stream

Fragment stream

Fragment stream

Vertices

Primitives

Fragments

Pixels